This invention relates to a piston for an internal combustion engine.
A conventional internal combustion engine employs a crankshaft to convert the reciprocating motion of the piston(s) into output torque to propel a vehicle or act upon any other load. The crankshaft is inefficient in its ability to convert the power available from the fuel combustion into usable output torque. This is because combustion of the fuel/air mixture takes place a number of degrees before the top dead centre (TDC) position of the piston, dependent upon engine speed and load. The ignited fuel/air pressure forces cannot produce output torque when the piston is either before or at TDC as the connecting rod and the crank pin are producing reverse torque before TDC and are practically in a straight line at TDC so that there is no force component tangential to the crank circle. This results in most of the available energy being lost as heat. If ignition takes place too early, most of the pressure generated is wasted trying to stop the engine (as this pressure tries to force the piston in the opposite direction to which it is travelling during the compression stroke); and, if left too late, the pressure is reduced due to the increasing volume above the piston as it starts its descent for the power stroke. The optimum maximum pressure point varies from engine to engine, but is around 12° after TDC on average.
The specification of my UK patent 2 318 151 relates to a piston and connecting rod assembly for an internal combustion engine. The assembly comprises a piston, a connecting rod, and a spring, the connecting rod having a first end operatively associated with the piston for movement therewith, and a second end connectible to a rotary output shaft. The spring acts between the piston and the connecting rod to bias the connecting rod away from the crown of the piston. The piston is movable towards the second (small) end of the connecting rod by a distance substantially equal to the cylinder clearance volume height. One result of using a spring is that the assembly has a resonant frequency, the advantages of which are described in the specification of my International patent application WO 00/77367. This assembly will be referred to throughout this specification as an energy storage piston.
In use, ignition is timed, by conventional timing means to take place at a predetermined time before TDC, so that the expanding gases formed by the ignition combustion force the piston to descend rapidly within the cylinder during the power stroke. Prior to reaching TDC, however, the pressure in the cylinder will build up to a high value, and the piston is forced towards the crank pin, against the force of the spring. This compresses the spring, and increases the volume above the piston, causing a reduction in pressure and temperature in the cylinder. The lowered temperature reduces radiation losses and the heat lost to the cooling water and subsequently the exhaust, with the pressure being shared equally between the cylinder clearance volume and the spring. This energy stored in the spring is released when the piston has passed TDC, and leads to the production of increased output torque. This is achieved as the spring pressure is now combined with the cylinder pressure after TDC. A large proportion of this stored energy would otherwise have been lost as heat, owing to the fact that the fuel/air mixture must be ignited before TDC, which is a result of the requirement for the ignited fuel/air to reach maximum pressure by about 12° after TDC for optimum performance.
One problem with the type of energy storage piston disclosed in the above-mentioned patent specifications is the necessity to have relative movement between the connecting rod small end and the piston crown in order to store energy in the spring arrangement mounted between these two parts. This problem has manifested itself in wear of the spring arrangement and/or adjacent parts, this wear being due to the failure of the assembly to maintain rigid axial alignment between the moving parts. This misalignment can cause heavy wear, and sometimes leads to seizures between adjacent parts, particularly when the piston is on fill load.
The specification of my European patent application 1274927 describes an energy storage piston that has improved alignment properties. This piston incorporates a spring which is integrally formed with the piston, and is configured as a bellows spring, and is made of titanium.
The disadvantages of this bellows spring piston are that it is difficult to manufacture, and can suffer from excessive stress forces if overloaded. Thus, if the bellows spring is manufactured from an annular block of titanium by machining internal and external slots, these cannot be done without computer numerical control (CNC), and this is a costly exercise as it requires a considerable time input to generate the correct cross-section of the bellows to achieve a functional piston. Moreover, the machining of the slots results in a considerable wastage of expensive titanium, and each spring will have to be specifically designed for a given piston and its application. Furthermore, because of the curved internal and external portions of the bellows spring and the requirement that the opposite faces of adjacent leaves of the spring must be contoured in order to spread the stress concentrations, the gaps between adjacent leaves are relatively large—of the order of 3 mm—and this leads to excessive stress problems if overloaded. Thus, a bellows spring is produced which has a relatively few leaves per unit length, and these must take up the large stress forces to which the piston is subjected in use. Accordingly, the stress per leaf is relatively high, and this can lead to premature failure of the spring. An additional disadvantage of this type of bellows spring is that, in order to attempt to achieve the required stress and deflection figures, it occupies a comparatively large space, making piston design difficult. Thus, space that is required for other piston components has to compete with the space occupied by the bellows spring. Throughout this specification the term “leaves” should be taken to mean those parts of a bellows spring that form the corrugations of the spring.
Alternatively, if individual leaves of the spring are formed by stamping, and the leaves are diffusion bonded together to form a bellows spring, a more cost-effective bellows spring can be produced, but this still suffers from excessive stress problems owing to the relatively large gaps between the leaves which are inherent in a bellows spring having curved internal and external end portions and non-parallel leaf walls. Space problems also occur for the same reasons as outlined above.
The specification of my UK patent application 0216830.0 describes an energy storage piston incorporating a spring acting, in use, between the piston and an associated connecting rod so as to bias the connecting rod away from the crown of the piston. The spring is configured as a bellows spring having a plurality of substantially parallel leaves defining the corrugations of the bellows spring. The internal and external end portions of the spring that connect the leaves are of rectangular configuration, and the gaps between adjacent leaves are defined by substantially parallel surfaces.
This spring has the advantages of being easier to manufacture than earlier types of bellows spring, and it does not suffer to the same extent from over-stressing. It does, however, still occupy a lot of space within a piston, which results in difficulties in piston design.
The specification of my UK patent application 0218893.6 describes a piston incorporating spring means acting in use between the piston and an associated connecting rod so as to bias the connecting rod away from the crown of the piston. The spring means is configured as a generally circular cushion spring located substantially in the region of the piston crown and extending over substantially the entire transverse cross-section of the piston, the spring means being such as to permit the crown of the piston to move axially relative to the connecting rod.
The disadvantage of this cushion spring is that it needs to be manufactured from two identical members whose edges must be bonded together. Electron beam welding is the preferred bonding method, but this process results in the material in the weld region being taken above its Beta Transus temperature, which results in the material becoming brittle, thereby shortening its useful working life.
The specification of my European Patent Application 1616090 describes a piston incorporating two disc springs within the piston, the disc springs acting in use between the piston and an associated connecting rod. Circumferential edge portions of the disc springs are supported and separated by a substantially annular support member, the springs being located substantially in the region of the piston crown and extending over substantially the entire transverse cross-section of the piston. The springs permit the crown of the piston to move axially relative to the connecting rod. The support member is constituted by respective rings fixed to the circumferential edge portions of the disc springs, and by an annular band formed with curved support surfaces for rolling engagement with the rings.
The disc springs of this piston are made of Titanium 10-2-3. The disadvantage of this material is that it requires at least two discs to achieve the desired deflection, and even then the full load stresses are close to the fatigue limit. This leads to a relatively short working life for the springs.
The specification of my UK patent application 2431451 describes a piston incorporating a disc spring made of a super-elastic material such as Nitinol. This spring is much smaller than the rectangular bellows spring, so that it can be fitted into the space between the piston crown and the top of the carrier. Moreover, being smaller, it uses considerably less metal, and so leads to a piston having a reduced cost. Furthermore, the springs can be located entirely at the crown end of the piston, and so enables the carrier to be made of aluminium rather than titanium which was the case with the improved rectangular bellows spring design, thereby leading to a further materials cost reduction.
This spring is also much lighter than the rectangular bellows piston; and, due to the simplicity of its design, its manufacturing process is more economical, faster and simpler. Yet another advantage is that existing piston designs can easily be modified to accept this type of spring, thereby permitting existing internal combustion engines to be modified to take advantage of the improved efficiency and fuel conservation properties of the energy storage piston.
Unfortunately, testing of Nitinol springs in an internal combustion engine revealed that they heat up internally during operation causing their premature failure.
The present invention is based on the discovery of a beta titanium alloy called gum metal (also known as TNTZ), which is a unique alloy of high elasticity, ductility and yield strength, originally developed with a composition of 54.3% titanium, 23% niobium, 0.7% tantalum, 21% zirconium and 1% oxygen, and can exist over a range of compositions which also include vanadium and hafnium. Gum metal exhibits a super-elastic nature one digit higher in elastic deformation (2.5%) compared to general metallic materials, has an ultra-low elastic modulus with high strength, has a super-plastic nature permitting cold plastic working to 99% or more with no work hardening at room temperature, has ultra-high strength of more than 2000 MPa by applying a heat-treatment, and has a near zero linear expansion coefficient (Invar property) and a constant elastic modulus (Elinvar property) over a wide temperature range